1. Field of the Invention
The present invention relates to a still image or moving image coding and decoding method, and more particularly, to a baseline-based contour coding and decoding method.
2. Description of Related Art
In a method of coding a still or moving image in object units, a contour coding with respect to an object plays an important role. In particular, in a low-transmission rate coding such as MPEG4, a burden followed by the coding of contour must be reduced as much as possible. For this, the contour must be highly compressed while being reconstructed similar to the original contour. Also, most contours has continuity, so it can be comprised of slowly-changing one-dimensional data. Therefore, the contours can be effectively utilized for the coding using transformation.
A general contour coding method is a chain coding method being widely used in a computer vision. In the chain coding method, a code with respect to each direction is previously defined, and a contour is coded using the predefined codes with respect to tracking directions while tracking the contour in a consistent direction. According to this chain coding method, information on the contour is completely preserved, so that the contour can be reproduced similarly to the original contour. However, a great amount of bit is generated due to the coding, and it is difficult to consider time redundancy in a sequence signal such as video.
An image processing field has a lot of application fields which do not require an extremely accurate contour reconstruction. In these application fields, the chain coding method generating many bits is not appropriate, so other methods, which generates a small amount of bit even though information on the contour is not completely reconstructed, are required. One method among them is a polygonal approximation method which approximates an entire contour associated with one object using a polygon. This polygonal approximation method is greatly effective in respect of bit generation, but deteriorated in reconstruction.
To overcome this defect, a spline approximation method has been proposed. The traditional polygonal approximation method approximates a contour to a straight line, while the spline approximation method approximates the contour to a curved line. Accordingly, a higher reconstruction can be accomplished due to the identical amount of bit.
Meanwhile, a discrete sine transformation (DST) instead of the spline method has been proposed. In this method, a contour is first approximated by the polygonal approximation method, one straight line of a polygon is sampled in a predetermined number, and then a sampling error value of a difference between the polygon and the contour is discrete-sine-transformed and quantized, thereby completing the coding process. According to this method, a complicate contour can be more accurately reconstructed even by a smaller amount of bit.
However, the several aforementioned contour coding methods are all methods for performing contour coding in still images. Each object in a moving image usually has a similar shape and a similar position within adjacent time, so that the contours of the respective objects also have time redundancy. Thus, efficiency of coding can be enhanced by coding the contour of a moving image using time redundancy.
According to an example of a method for coding the contour of a moving image, the motion of objects of a k-th frame is estimated depending on objects of a (k-1)th frame and then compensated, so that a contour to be coded is extracted from an unoverlapped portion between the k-th and (k-1)th objects and the extracted contour is coded by the chain coding method. However, in this method, a contour is coded to a lot of short-length lines when the contour of a moving object is complicated, which results in a frequent coding of a start point coordinate of the contour. Thus, the moving image contour coding method is inefficient.